Multifrequency oscillator



May l1, 1943.

R. C. FISHER MULTIFREQUENCY OSCILLATOR Filed April '7, 1941 4Sheets-Sheet l @nymond C ffe//V Uttorncgl May 11 1943' R. c. FISHER2,318,936

MULTIFREQUENCY OSCILLATOR Filed April 7, 1941 4 Sheets-Sheet 2 FIG o ZTo lnPuJr 0'? Lmplnfier 52 '7 Bluwntor Gtforncgl -Ml' 11 l943. R c.FISHER MULTIFREQUENCY OSCILLATOR Filed April '7, 1941 4 Sheets-Sheet 3UU |5157 MEZ 010|@ a a 7m m Z n w .7 2/

4 5 5 5 mL. 2 5 C FIG l2 no6 no4 nos n, ,o

U Ba Ga May 11, 1943.

MULTIFREQUENCY OS C ILLATOR R. c. FISHER 2,318,936

Pima April '7, 1941 4 Sheets-Sheet 4 FIG I8 :Inventor Ctttorncg PatentedMay 11, 1943 UNITED STATES PATENT OFFICE MULTIFREQUENCY OSCILLATORRaymond C. Fisher, Tacoma, Wash.

Application April 7, 1941, Serial No. 387,270'

(Cl. Z50-36) 20 Claims.

My invention relates to the art of generating periodic electric currentsby means of resonant oroscillatory electrical or mechanical means incombination with an amplifying electron tube system adapted to maintaincontinuous oscillation in such means.

It has particular application to the art of generating periodic currentssuited to the production of musicaltones.

One object of my invention is to provide generating means of the classset forth above, adapted to produce a number of different frequenciessimultaneously without a correspondingly large number of electron tubeampliiiers.

Another object is to provide a type of multifrequency generator oroscillator having amplitude and frequency stability sufficient formusical applications without undue maintenance difcultiesor complicationin the apparatus.

A further object is to provide an individual means for limiting theamplitude of oscillation of a plurality of resonant or oscillatoryelectrical or mechanical elements, all of which plurality are maintainedin continuous oscillation by means of a common amplifier, without at thesame time introducing mutual interference between the several resonantelements.

Another object is to provide novel means for varying or modulating at asubaudible periodic rate the frequency produced by such resonant meansfor the purpose of vibrato effects.

Another object is to utilize the resonantA properties of a sharply tunedelement to effect an organ-like attack and decay of tones.

Another object is to provide a coupling means through which amechanically tuned or resonant element may drive or actuate anothertuned element in a source of potential, the secondelement beingresonantto the same frequency asy the first orto amultiple thereof.

Still another object is to prevent leakage of electric current betweenthe mutually vibratile electrodes of a potential-generating condenser.

Other and further objects will appear from the following description andthe accompanying drawings, among which: Y

Figure l is an assembly of vibratile or oscillatory reeds, adapted foruse in a musical instrument, together with means for maintaining thesame in vibration and means fory limiting the amplitudes of vibration ofthe individual reeds.

Figures 2 and 3 show means for producing vibrato effects in connectionwith the reeds of Figure l.

Figure 4 shows details of an electrostatic means for picking up thevibrations of a reed.

Figure 5 shows vibratile reeds together with means for mechanicallycoupling the two together.

Figures 6 to 10 inclusive show forms of electrostatic pickup from a reedwhich are adapted to limit the amplitude of oscillation of said reed.

Figure 11 shows a purely electrical means for conilishing the samegeneral result as Figures Figure 12 shows an acoustic means foraccomplishing the same general result.

Figures 13 and 14 show an electrical network adapted to actuate a groupof secondary reeds from a set of primary tone generators.

Figures 15 and 16 show resonant circuits maintained in continuousoscillation by means of an amplifier, together with means for limitingthe amplitude of oscillation of each of `said circuits.

Figure 17` shows a supersonic-frequency network whereby a plurality ofmechanically resonant members may be simultaneously kept in continuousoscillation and limited as to amplitude.

Figure 18 shows details of magnetic means adapted, in conjunction withparts of other figures, to limit the amplitude of oscillation of amechanically resonant member.

Suppose it is desired to generate a large number of frequenciessimultaneously, such as the ninety-six one might require for eightoctaves of a musical instrument. Naturally, the means for accomplishingthis should be as simple as possible. To this end, I should prefer tomake a single amplifier serve incommon for all of them, or at least foras large a part of them as may be. In the usual single-frequencyoscillator, the one or more tubes of the amplifier itself function toylimit the amplitude, by virtue of its grid being driven positive. SeeResistance Stabilized Oscillators, by F. E. Terman, Electronics, July1933, published by McGraw-Hill Publishing Co., Inc., New York, N. Y. Inthe multi-frequency oscillator, however, the common amplifier cannot ingeneral do this successfully. Its grid circuits are subjected to a largenumber of different frequencies, each associated with a different one ofthe multiplicity of oscillatory circuits, and if a potential componentat one of these frequencies drives the amplitude-limiting grid positive,then potentials at all the frequencies will be. limited simultaneously.The result is generally that some components, perhaps all but one, areso suppressed by the automatic limiting action, that only one of them,or at best only a few actually appear. Moreover, the non-linearitypresent in the amplifier results in a large number of sum and differencefrequencies, which tend toward frequency instability and interlocking ofthe various frequencies. For critical applications, such as a musicalinstrument, this state of affairs is nautrally to be avoided.

It will be evident that a common amplifier for al1 of the multiplicityof oscillatory circuits and frequencies is usable only provided it issuiiiciently free from non-linearity, and provided one provides aseparate automatic amplitude limiter associated with each such frequencyand oscillatory circuit, and so arranged that it has no appreciableinfiuence on the amplitude or frequency associated with any of the otheroscillatory circuits. Each limiter must be responsive solely to currentsor potentials of its associated frequency, and it must be capable oflimiting its associated current or voltage to a magnitude low enough sothat the amplier will not be overloaded by the sum total of all thepotentials which it must amplify. Amplitude limitation must beaccomplished otherwise than by the positive potentials in a grid circuitcommon to the several frequencies. Otherwise, the amplifier cann-ot besubstantially linear, as above postulated.

In case a common amplifier is to be thus employed, one should, toexpress the matter in general terms, provide a separate feedback channelfor each such frequency, each of these channels having a resonantfrequency-determining means (in the above case the oscillatory circuit)and also an amplitude-limiting means individual to it and to itsassociated frequency. Where the number of frequencies to be generated isgreat, I can in general aiord to build a fairly elabcrate amplifier,since it may serve for all the frequencies. n the other hand, since thefrequency-determining means and the amplitudelimiting means must beduplicated for each frequency desired, I should prefer to keep these assimple as possible, even though to do so might necessitate a somewhatmore elaborate amplifier.

The above discussion has been carried out with reference to anoscillatory circuit for the determination of each frequency. However, itis well known in the art that various other sharply resonant or sharplytuned frequency-determining agencies may be used, such as metal bars,bells, strings, reeds, tuning forks, or other such mechanically resonantelements. In addition, piezoelectric or magneto-strictive elements maybe similarly used with suitable amplitude-limiting means. Any of theseelements may be maintained in continuous oscillation or vibration bymeans of an electron tube amplifying system, as

known in the art. My invention comprises all such suitableelectro-mechanical or purely electrical means for determining thefrequency.

Because of its unusual simplicity, purity of wave form, and stability ofgenerated frequency, however, I prefer to use as the resonant element anoscillatory ferromagnetic reed, illustrated at I in Figure l, whichforms a part of a reed generator assembly 26. The remainder of the igureis one particularly well suited for use in an electronic musicalinstrument, but it will be apparent that it may have other applicationsas well.

The reed is suitably supported by a rather massive casting 2, beingelectrically insulated therefrom by means of insulating Bakelite washers3, and by means of Bakelite or hard rubber sleeves around the mountingscrews. These sleeves are not illustrated, being similar to thosecommonly used with relay springs and key contacts in the communicationart. The reed is held at all times at a suitable potential with respectto ground by a source of direct polarizing potential 6B, being connectedthereto Via conductor 27 and resistor 4.

At I9, I show an adjustable electrode, supported by casting 2 andinsulated therefrom, as shown. The electrode has a disk-shaped lowerextremity, providing an appreciable electrostatic capacitance withrespect to the reed. The electrode and reed thus constitute twoelectrodes of a variable condenser or capacitor, whose capacitanceperiodically increases and decreases as the reed oscillates. Inconsequence of this variation, and of the polarization provided by thesource Se, an alternating potential at the frequency of the reedvibration will appear at the input terminals of amplifying electron tubesystem This amplifier may have as many stages of amplification as areneeded to enable it to perform its function, as will appear. Theamplifier being suitably linear, as above postulated, the same frequencyand substantially the same Wave form will appear at the output terminalsof the amplifier. A circuit connection is provided from theplate-connected output terminal via conductor 28', through certainmagnets not illustrated, via conductor e, through elements (to be laterexplained), conductor I6, resistor 35, through eleven of the elements 24to a terminal M of electromagnet 2i. From the magnet, the currentreturns via conductor 21 to the B-battery 6?. magnet 2i is in the platecircuit of the output tube of the amplifying electron tube system 32.Inasmuch as the magnetic path of the magnet is chieiiy in air, its iiuxwill be substantially proportional to the current traversing itswinding. Since the pulsations of flux originate at the reed, they willhave the same fundamental frequency as that of the reed. Hence the reed,being tuned' to this frequency, may respond strongly.

Mathematical calculation will show that, by proper control or choice ofthe condenser 23, grid leak 8, variable impedance which is shuntedacross the several elements 2li, and of other coupling impedances of theamplifier, not shown, the magnetic force impressed on the reed may bemade to be degrees out of phase with the velocity of the reed ;V inother words, with the force impressed by the mechanical resistance ofthe reed. The elements 23 and 8 should also be chosen so as to renderthe amplifier input network quasi-linear, as explained in my Patent No.2,055,719.

Now, mechanical resistance is analogous to electrical resistance, andthe reed vibration may be caused to build up in amplitude by making thefundamental component of force contributed by the magnet somewhatgreater than that dissipated or expended in the mechanical resistance.

Some way must, however, be provided to limit the amplitude of vibrationto some chosen value,

as discussed hereinabove with respect to the oscillatory circuit. Thisfunction is performed in Figure 1 by contact spring I2 and contactadjusting screw I8, insulated from the contact by its insulating tip I5.As soon as the reed attains a certain amplitude of oscillationdetermined by the setting of screw I 8, the reed makes periodic contactto the spring at each upward swing. During each period of contact,` thereed is partially grounded through resistor 5, casting I2|, the twoscrews at the lower left-hand part of In short, the

the casting, the electrostatic shield 22, and the ground connection 80.

As has been previously pointed out, the reed is maintained at apotential above that of ground by virtue of its connection to a sourceof polarizing potential 60. There is a condenser 89, connected betweenthe reed and ground. Because of this, the reed potential cannot bealtered instantaneously.l By virtue of the voltage-dividing action ofthe resistors 4 and 5, however, the average potential over a cycle willbe less` the greater the amplitude of reed vibration, because for largeamplitudes the contacts at I2 are made for a greater fraction of a cyclethan for small amplitudes. By proper choice of the electrical magnitudesofrresistors 4 and 5 and condenser 89; the time constant of the networkwherein they appear can be made considerably greater than one period ofthe reed vibration, and the polarizing potential of the reed will remainsensibly constant throughout a cycle.

The alternating potential impressed upon the input terminals ofamplifier 32 is directly proportional to the polarizing potential. Thus,lowering the polarizing potential effects a decrease in what I here termthe electron tube system input transduction factor; that is,A it lowersthe ratio between the amplitude of fundamental component of amplifierinput potential and the fundamental component of reed velocity. It isclear, then, that the reed will attain an amplitude just suicient topermit the amplifier to exactly overcome the reeds mechanical losses,and that it will hold this amplitude until some change occurs in themechanical or electrical conditions,

such as a change in amplifier B potential orthe like. In the lattercase, the reed amplitude may change slightly to accommodate itself tothe new conditions. It will be seen, however, that the amplitude may bemadevery stable indeed by proper adjustment of variable resistor 5,adjusting screw I8, magnet core 45, and the usual attention to thesupply potentials, etc.

The *magnet core is threaded inside an insulating sleeve, not shown,which lnsulates it from the winding in the usual fashion, and has aslotted head, as illustrated, to facilitate accurate adjustment.

In order that the reed itself shall be the determining factor as regardsfrequency, the contact spring I2`should be made very light in weight andits force against the reed should be negligible as compared with thestiffness forces of the reed itself. If these conditions are fulfilled,if the amplifier is linear, and if the amplitude of the reed issufl'lciently small as compared with the minimum distance from it to thetop of magnet 2|, and from the electrode I9, then the reeds motion willbe nearly enough sinusoidal for all practical purposes, and the outputpotential of the amplifier 32 will likewise be so.

VWith certain amplifiers, it will be seen that the electrode I9 shouldbe beneath the reed instead of above it, in order to provideregenerative feedback. In such case, the electrode might be convenientlymounted on the upper end of the core of magnet 2 I, or be made integraltherewith. To prevent electrostatic coupling between the magnet windingand the core and electrode, electrostatic shielding isolating thewinding from the core and electrode would be advisable. This could besimilar to the split shielding commonly used between windings oftransformers in the radio art. A similar shield might also be used ifnecessary between winding and core in .the

magnetsof Figures 3 and 18, as will appear.. The shield in every suchcase should beY grounded.

The, contact spring, I2 need carry only a small current at any time;scarcely enough, in fact, to produce visible sparking, provided a smallenough condenser is used at 89. The destruction of the contact pointwill be very much less than though it directly controlled the current inthe magnet 2|, as in the conventional contact-controlled tuning fork.Moreover, the wave form of current where contacts are used would be toofar from sinusoidal for use in the present environment. If desired, onecontact point may be placed in the reed at the point where theadjustable Contact I2 touches it, and both may be of platinum-ridium orother noble metal to avoid contact corrosion or oxidation. The insertedcontact point does not appear in Figure l, since it may be flush withthe top of reed I. However, there may be considered to be two contactpoints, one on the reed and one on the spring, although one of these maybe an integral or non-integral part of the reed proper.

If the amplifier impresses upon an oscillatory circuit any potentialcomponent in quadrature with the current in that circuit, and if thecircuit is rendered self-oscillatory by the amplifier, then the presenceof the quadrature component will produce a change in the frequencygenerated. A similar phenomenon will occur in the case of the reed orother mechanically oscillatory element. In accordance with theprinciples enunciated in Chapter IV, of Elements of AcousticalEngineering, by Harry F. Olson, published by D. Van Nostrand Co., Inc.,250 4th Ave., New York city, reed velocity is analogous to current inthe oscillatory circuit, and a force impressed on the reed which is inquadrature with the velocity 0f the reed will thus have the effect ofaltering the frequency of the reed. By a variation of the impedance ofthe element 35, the phase relationship between amplifier input potentialand potential drop across the magnet 2l may be varied. 'The impedance 35may be a resistance, a capacitance, an inductance, or any combination ofthese which may be suitably varied. It is clear that, if it be variedperiodically at a subaudible frequencf,J of from, say, five to tencycles per second, and if the variation in reed frequency and pitchvaries accordingly, the resultV so far as the ear is concerned will besimilar to that of the vibrato of a musical instrument. A suitableconstruction for thisimpedance will be suggested hereinafter.

It has now been shown how a single mechanically oscillatory or resonantJreed, together with an amplifier and other associated equipment, may bemade to continuously generate a sinusoidal current or Voltage ofconstant amplitude. I now propose to show how other reeds and associatedequipment, maintained in continuous oscillation by the same amplifier,may be employed to simultaneously generate other frequencies, withoutmutual Interference- The reed I above discussed was apart of a reedgenerator assembly 24, and

constituted a part of a channel adapted to feed back potential from theoutput terminals of the amplifier to the input terminals thereof. We mayassume that this reed is so constructed that its assembly maintains afrequency of 32.7 cycles per second, which is the frequency of lS-footC. In order to give the reed a sufficiently low pitch. it isconstructed` with a rather massive tip, as shown. I may wish also togenerate the other eleven semitones of the l-foot octave, from C# to'Binclusive. y l other assemblies 24, each in an individual channelbetween the output and the input terminals of the electron tube system.These other eleven are shown merely as boxes, although it is to beunderstood that they are exactly like the assembly 24 shown at the upperleft-hand coiner, except in such respects as may be required to enableeach to produce and to continuously maintain its associated singlefundamental frequency. For example, in general the higher the pitch, theshorter and lighter is the reed for its generation.

It will be observed that each of the assemblies has its individualamplitude-limiting means in the form of a contact spring I2 andassociated parts. The twelve magnets 2l are connected in series asshown. I wish to make it especially clear that each of theamplitude-limiting means is adapted and electrically connected tocontrol and limit the amplitude of oscillation of its associated reedand of the potentials and currents associated therewith, but not theamplitudes associated with any reed among the other eleven present. Ialso wish to point out that the amplitude-limiting means is responsiveto motion or current of but one frequency, because it is responsive tothe amplitude of its associated reed only, and that reed is so sharplytuned as to be virtually unaffected by the presence of the other elevenfrequencies in the exciting current in its magnet 2l. Thus I haveprovided; (a) a plurality of frequency-determining elements, namelytwelve reeds, (b) a plurality of amplitude-limiting means, eachresponsive only to oscillations having the frequency maintained by thatchannel of which that means is a part, and adapted to limit theamplitude of oscillations of that frequency alone, to the exclusion ofthe other eleven, and (c) a common amplifying electron tube systemsubstantially free from non-linear distortion, adapted to be excited byall of the plurality of frequency-determining elements by beingconnected at its input and output terminals to the severalelectro-mechanical channels and adapted to continuously maintainsinusoidal impulses in all of the several frequency-determiningelements, each at its associated resonant frequency. The severalfrequencies thus maintained coexist simultaneously, and the frequency ofpotential and current maintained in the amplifier by a given reed issubstantially the same as the natural frequency of that reed.

I have stated that each of the twelve reeds I is to be stronglyresonant, in order that the twelve shall not interfere with each othersbehavior. The ratio between two frequencies a semitone apart is roughlysix per cent. Hence in a musical instrument it is necessary that eachreed shall be sharply resonant enough so that it is virtuallyunresponsive to a frequency different by six per cent from .its resonantfrequency. This is a stipulation not very difcult to realize in practicein the sixteen-foot octave, and I have found by experiment that tworeeds tuned a semitone apart will operate very successfully inconjunction with the same actuating amplfier provided they are notclosely intercoupled mechanically by being mounted on the samesupporting frame 2, or the like. Although their electrical outputcircuits are connected together to the input terminals of the amplifier,yet the mechanical load which such high impedance circuits impose uponthe reed are very slight indeed, and the inter-reed coupling resultantfrom To this end, I provide eleven such a oommon'electric connection maybe expected to be very slight indeed. However, if it'should be foundthat, under some other conditions than those which I set upexperimentally, there should be appreciable or serious interference, itwould be within the scope of my invention to associate six reeds withone amplier, and the remaining six with a second amplifier. For example,the reeds for generation of C, D, E, Fit, Gt, and Asi-of thesixteen-foot octave might be actuated by the first amplifier, and theCt, Di, F, G, A, and B reeds of the same octave might be actuated by thesecond one. In such case, the frequencies of any two reeds Vassociatedwith the same amplifier would differ by more than eleven per cent. Orthe twelve generators might be divided into three or four groups, eachgroup having a separate actuating amplier.

Besides the twelve generators for the twelve tones of the sixteen-footoctave, a complete musical instrument should also have generators fortones of higher octaves. At 25, I illustrate six such generators. Theremay be as many as necessary for the gamut of tones desired. All of thesemay be constructed similarly to each other and to generators 2li, exceptfor the dimensional differences necessary to adapt each to the pitch itis required to generate, and except for other differences set forthbelow. I therefore consider it sufficient to illustrate only one indetail in Figure 1, and to illustrate others merely as boxes, omittingstill others altogether. Like parts in generators 24 and 25 bear likereference numbers. It is desirable that the generators for the octavesabove the sixteen-foot octave shall each produce a frequency or pitchbearing a precise octave or multiple-octave relationship to itscorresponding generator of the sixteen-foot octave. To this end, I haveshown in one of the generators 24 a frequency-doubling electrode 20,insulatingly supported from casting 2. Due to its placement relative tothe reed I, it will be seen that electrode 20 will have a capacitancerelative to the reed I which passes through a maximum value twice duringeach cycle or complete vibration of the reed; namely,wlien the reedpasses through its position of rest or equilibrium. Moreover, due to therelative dimensions and placement of reed and electrode, virtually noodd partials of the fundamental reed frequency will appear in thepotential which this electrode supplies to the amplier 32, and only evenpartials will be present. Among these even partials, the second partial,or octave partial, will have the greatest amplitude; next to that of thesecond will be the amplitude of the fourth partial, and smaller stillwill be that of the sixth, and so on. Since the fundamental reedfrequency is assumed to be 32.7 cycles for the generator illustrated indetail, the second partial or second harmonic frequency will be 65.4cycles.

Electrodes I9 and 20 being connected together and to the input terminalsof a common electron tube system 32, they are equivalent to a singleelectrode producing harmonic distortion and a complex wave form of inputpotential. This input potential, after amplifier 32 has suitably ampliedthe same, will appear in the current of magnet 2|' which forms a part ofone of thegenerators 25. Most prominent among the components of thatpotential will be the first harmonic or fundamental, and the secondharmonic. The reed I' which appears in the figure is assumed to be tunedto the octave frequency of 65.4 cycles, which is the frequency ofeight-foot C. Being gaat@ coupled to reed I through the amplifier andthrough magnet 2|', reed I Will respond sympathetically to thatfrequency. It is to be understood that the generator 25 has no means forsustaining or limiting the amplitude of its own vibrations. Itsoscillations are entirely forced, not self-maintained, ones, and aresustained continuously whenever reed I is oscillating. There is nonecessity for self-maintenance or self-limiting in generator 25, sincethe frequency and amplitude of reed I are entirely determined by thefrequency and magnitude of that current component which was supplied tomagnet 2 I by reed I. Of necessity, then, reed I' is at all4 times inoscillation at precisely double the frequency at which reedl oscillates.Y

At I provide a pickup electrode insulatingly supported from the casting2, like electrode 20 of generator 2li. It will be evident from thesymmetry in the relative shapes and placements of reed I' and electrode23' that this electrode will supply to amplifier 32 a wave fornihavingonly even partials of the 65.4-cycle frequency of reed I'. Just aselectrode 20 is able to supply to mag: net 2l the frequency neededformaintenance of the reed I', tuned an octave labve reedf I, soelectrode 20 is able to supply to the magnet of another of thegenerators a frequency of double 65.4 cycles, or 130.8 cycles,whichisthat of C in the four-foot octave. I haveconsidere'd itunnecessary to illustrate the last-mentioned generator, or others of itsown or higher octavos. However, it is to be understood thatsuchgenerators are to be constructed similarly to the generator 25illustrated, except that' their, reeds are to be tuned to the respectivefrequencies and constructed accordingly. Their driviflgor actufatingmagnets 2I' are to be ,conr'ieztedfA the output circuit ofamplifier 32 between ecnductors 28 and 59, at the point marked To othergroups of magnets 2I.

I havev explained that one of the generators or sources 25, thatillustrated in detailis actu-4 ated by currents supplied toit `vthroughthe amplifier 32 by one of thegenerators 2 4.v ,The output terminals ofsource 24 areaecordingly connected through the common amplifier` @Ltoinput terminals of source 2 5.A `Due to distortion or harmonicproduction in the mechanicoelectric translating means, namely electrodes`IS Aand 2. Source 2&4 is enabled t0 Supply, the. frequency necessaryyto sustain vibra-tion in a` generator reed lziuned an octave abovethereeel.m limite manner, this last-mentioned reed vand generator inturnproduce a twice-doubled frequency for the maintenance of ,anotherreed, I', omittedfrom the figure as superfluous vso far as purposes ofillustration are concerned. l H g 121150 illustrate flve othergenerators L2i, shown merely as boxes Each is adapted, Jlikethegenerator .25 Ywhich appears .in detail. tdbe actuated by one of thegenerators 24, vo f corref spondingpitch name in thesixteen-footvoctave. Thus, the C# generator 25 is, actuatedby currentsupplied from the C# generator 24, and so, on. Moreover, each of` ,thegenerators 25 isv adapted to supply actuating current` atY the requisitefrequency for a generator vof `corresponding pitch name in thefour-,foot octave. Forrreasonskobvlous from the foregoing, eachof thegenerators 24 may be termeda master generator and each of the generators25, whatever theoctave in which the same occurs, may be terniedav slavegenerator.

This process of actuating the corresponding one of the next, loweroctave can bev carried on ythrough several octaves merely by providingsuitable connections to magnets, reeds, shieldsand pickupelectrodes ofall the generators as will be evident from the legends at the lowerl'efthand corner of Figure 1, and 'at the centerof `said figure. .Y t

Ifo make` matters perfectlyclear, I ,wish to point out that, from twelvemaster generators 24 and sixty Y slave, generatorsV 25, I may producefrequencies from sixteen-foot C to three-,inch B inclusive in theamplifier 3,2. rl'here .vvi1l be seventy-two generators in .all, whosereed frequencies range fromsixteenffoot Cy to six-inch B inclusive. Eachofr thevtwelve master reeds I ofthe first or sixteen-foot octave has afundamental-producing electrode I9, anda frequency doubling electrode20, and the group of twelve master generators is thus capable ofproducing at theV output` of amplifier 32 theV frequenciesfromrsixtefenfoot C toeight-foot B inclusive.

,'I'lieremainingsiXty reeds I have associated with' inem' 'nofundamentaleproducing'electrodes, but only frequency-doubling electrodesv2II'. Theyare tuned to the range fromeight-footrC to Asiii-inchBinclusive, and thus they generate double these frequencies, or thevfr'eque'nciesof the range. fr'o'ni four-foot Cto three-inch Binclusive. Before theto'nes reach the loudspeaker, further deutung mayoccur as will be; explained belovv, 'and hence the speaker may emitanaddi-kl tional octave oitoes, ranging up to 11/2-inc1iB.

It is t`oV be observed that the severity-two reeds I and V4I are allmaintained in continuolsvibration by thel electron tubes'ys't'einwheneverr the musical instrument is to be ready for `ifnfnediateusewheth `er any `tone is, actually being sounded by the loudspeaker ornot. rIf any of these reeds were to stop ,its vibration, all thosevibrating at frequencies exactly one'` or more octaves above that oftle' reed',inV question would soon cease to vibrate also, because theirrmagnetic driving forces wouldV haveA disappeared. ThusY itwould beout"of thequestio'n with the arrangement of Figure 1 to playk the tones atwill vrfrom .akey-V board by starting and stopping'thevibrationof thereeds I and If..v esidesfthis', the ordinary reed is an eitremelysharplytuned ror'stroiigly resonantvibratile element, especially in thelower o ctaves. Accordingly if one were to attempt to startand stopself-m"aintai`i1edl reeds, such as the reed I, by closing an electricconnection associated .with` it through the intermediary of a key,v itwould be found that the reed would probably attain its full amplitudevery slowly indeed, as pointed out in connection withelectromagneti'callymaintained tuning forks by Severy. Patent No.2,155,741, page, 1last paragraph. The response might `be considerablyaccelerated by providing the reeds with considerable damping, as asolution of their differentialequations will show. However, I prefer tocontrol the tones from the vkeys in a manner discussed below.

At the lower rightfhand corner of Figure 1, I show six slave generators26. One of these appears in detail, and the other five may beidenticallycenstruction exgeptfor such dimensionaldifferencesasare,necessary to adapt each to its required frequency ofoscillat ion. It will be seen that. eac h has an electromagnet 2i,andthat allof ,these are connected, in series in the output circuit @fampiiep 32. iso in `series with these may be themagnets of as many othergenerators l m2541101; Shown) .as may be needed to covervthe eachgenerator from gamut of tones'desired from the instrument,

The electrostatic shields of all are to be grounded by the conductor 85,as shown.

Present in the magnets 2l" are all frequencies from sixteen-foot C toand including three-inch B, or eighty-four frequencies in all,originating at the generators 24 and 25. I provide eightyfour slavegenerators 25, each having a reed I" tuned to one of the aboveeighty-four frequencies. Each reed will thus be kept in continuousoscillation. In each source 26 I also provide a pickup electrode I9"adjacent to the reed and insulated therefrom. In other respects, theoscillation generator 25 is quite similar to master generator 24, and itis accordingly believed that further detailed description of the partswill be superfluous. Like parts bear like reference numbers in the twogenerators, except for the distinguishing primes.

Suppose for the sake of deniteness that the reed I illustrated in Figure1 is tuned to 65.4 cycles. to amplifier 32 by electrode 20 shown nearthe upper left-hand corner of the figure. At the electrode I" may thenappear the same frequency, since there is no frequency-doubling actionin- It will then respond to the frequency fed i herent in itsconstruction. However, this frequency will appear only if there is adifference of potential between elements I and I9. The means forproviding this will be explained presently. Incorporated in each of theeighty-four generators is an electrode I9" like that illustrated, andall are connected together and to the input of amplifier 56 by means ofconductor 3| and condenser 9. The resistances 8 and Ill and condensers23 and 9 at the input of this amplifier constitute a coupling networksimilar to that illustrated in my copending application S. N. 379,287.It will sufce to remark here that condenser 9 is a blocking condenser,I0 is a grid leak, and that elements 8 and 23 may have electricalmagnitudes chosen so as to preserve quasilinearity as explained in theafore-mentioned application and in my Patent No. 2,055,719. In addition,a proper choice of electrical magnitudes of the four elements of theinput network and a suitable adjustment of electrodes I9 relative totheir respective reeds I may serve to lend a suitable frequency-loudnesscharacteristic to the tones ofthe loudspeaker. The amplifier 66 may haveas many stages of amplification as required, andv these may be of anytype suitable to the purpose, as well known in the communication art.The details of the amplifier itself do not constitute a part of thisinvention. Connected to the output terminals of the amplier I mayprovide a loudspeaker, not illustrated, the method of connection beingfamiliar in the art. 'I'ogether, amplier and speaker and theirassociated coupling arrangements comprise a network adapted to utilizethe electrical output of secondary'generators 25.

It is desired that no alternating potential shall be produced by theelectrode I9" except when its corresponding pitch is selected bydepressing a key of a keyboard, or the like. To this end, it isnecessary that the network connected to elements I and I9" be such as toimpose no polarizing potential difference between the two eX- cept whendesired. Besides this, it is desirable that the adjacent surfaces of thetwo elements be coated with thin layers of some chemically inert,electrically conducting substance, such as graphite. I suggest coatingthem with the graphitic substance known as Aquadag, or else sprayingthem with a thin layer of varnish or the like,

and then dusting on graphite powder when the varnish has become tacky.In my copending application S. N. 355,792, asimilar coating wassuggested for a similar purpose on condenser electrodes havingunidirectional instead of vibratile relative motion, and attention isdirected thereto.

To polarize the reed I from keyboards as desired for playing purposes,I' provide a. network comprising parts f5, 7, 5a., 6b, and 6c. Thisnetwork is to be connected to the keyboards by means of the conductorsIta, ISD, and I 3c, as illustrated and described in my copendingapplication S. N. 379,287, which should be referred to for a completedescription. The polarizing networks disclosed in Patent No. 2,216,513to Hammond might be substituted, as will be understood. The keyboard orkeyboards serves to connect these' conductors to a suitable source ofpolarizing potential, as will appear from either the cited patent ofapplication. Since the abovementioned elements 5, 7, 6a., 6b, and 5calso appear in figures accompanying application S. N. 379,287, bearingidentical reference numbers, it is believed superfluous to illustrate orexplain the keying network in detail here. Its details do not form partof the present application.

As shown by the legend at the lower right-hand corner of Figure 1, onenetwork consisting of elements 6, I, 5a, 6b, 5c, I3a, ISb, and |30 willbe needed for each of the generators 26, the present elements I and I9"being connected similarly to the stator and rotor electrodesrespectively of the application cited.

I have already remarked that frequency doubling can be effected in theslave generators 25. It is also advantageous to so construct certain ofthe secondary slave generators 26 as to permit them to generatefrequencies twice as great as the frequencies of their respectiveoscillatory elements. I may, as previously stated, provide eighty-fourpri-mary generators 24 `and 25, and also eighty-four secondarygenerators 26, the latter group having constructions and dimensionssuitable for oscillation at frequencies from sixteen-foot C tothree-inch B inclusive, and having electrodes I S" which do notintroduce frequency doubling. The air gap between reeds I" andelectrodes I9" should also be such as to initiate relatively littleharmonic distortion or upper har'- monic production, as hereinbeforeexplained. In addition, twelve secondary generators will preferably beprovided for generation of the twelve pitches of the 11/2-inch octave.The reed of each would be tuned to a pitch or frequency of thethree-inch octave, thus obviating the -need for exceedingly small andlight reeds. Adjacent to each reed would be an electrode like 20 ingenerator 25. This would double the frequency of the reed, and thus thetones produced by this uppermost octave of generators would lie in the11/2- inch octave, as desired. All twelve such secondary generators ofthe highest octave would be constructed identically with generators 25,except for the requisite dimensions needed to adapt them to the desiredfrequencies of oscillation and pitch of 'output potential.

It is within the scope of my invention to interchange the electricalconnections to the reed and electrode in generators 25 and 26, in whichcase the reed would be connected to grid and the electrode to polarizingpotential. A certain economy could be elfected in this way in that asingle reed I" could be made to feed to amplifier 66 two different`frequencies, one in the three-inch and one in the 11/2-inch octave. Toaccomplish this, one might provide, adjacent to a reed I", an electrodeI9" for generation of the reeds fundamental frequency, and also a secondone like electrode in generator 25, for generation of twice thefundamental frequency. Either frequency could then be had from a singlereed merely by polarizing `the appropriate electrode.

For obvious reasons, the generators 24 and 25 are termed primarygenerators, while the generators 25 are known as secondary generators.It may be wondered why I have provided a primary set and an independentsecondary set. This was not strictly necessary, and could be avoided byproper insulation, construction, and circuit connection, as will beevident to one skilled in the art. However, it was done in order thatthe primary set might be kept continuously .polarized for themaintenance of continuous vibration, while the secondary set might lbepolarized only as they were to be played into the loud-speaker. It willbe evident that a single set would have served both primary andsecondary purposes provided the reeds were kept grounded at all times,and provided polarizing potential above ground were to be applied to thepickup electrodes, these same electrodes then serving also for feedingthe alternating potentials to the amplifiers. Associated with each reedwould be at least two pickup electrodes, one connected to amplifier 32and the other to amplifier E6, the former being kept polarized at alltimes, and the latter being polarized only as its associated tone was tobe played.

I prefer the disclosure illustrated inFigure 1, however, because thesimpler one just described would doubtless have certain disadvantages.It would be more difficult to avoid key clicks when polarizingpotentials were applied to those `electrodes connected to amplifier 65,or when they were removed therefrom. The electrodes associated with thereeds of higher pitch would have to be quite close together for reeds ofthe usual dimensions, and careful electrostatic shielding would have tobe provided. Otherwise, inductively produced tones might be heard in theloudspeaker even when no key is pressed. However, the simplerarrangementis not to be regarded as inoperative.

I have remarked above that the physical structure of variable impedance35 in Figure 1 would be explained later. In Figure 2, I show one way inwhich such an impedance might be made. At 4I I show a conducting segmentand at v42 an insulating segment in a rotating drum, as shown. The drumcomposed of these parts may be rotated by an electric motor, or by anyother suitable means, not shown. At 4i) I show a carbon brush adapted tomaintain continuous contact with the inner portion of segment 4I. At 39I show five carbon brushes arranged at suitable intervals around theperiphery. When ,only one brush 39 is in contact with the segment,onlyone circuit connection will exist through the device betweenterminals 36 and 31 via one of the condensers or other impedanceelements 38; later, there will be two paths in parallel, asa secondbrush comes into contact; still later, there willbe three, etc. Bysuitably choosing the electrical magnitudes of the several impedancelelements 38, and by suitably locating brushes 39, the wave formrelating impedance and time may be controlled. That decided upon willdoubtless depend upon the designer. The drum will preferably berotatedat from veto ten revolutionsper second.

The frequency of a reed or other mechanically vibratile member isproportional to the square root' of the ratio of its stiffness to itseffective mass. If the stiffness is increased, the frequency will beincreased. Now, the stiffness exerts a force tending to restore the reedto its position of rest or equilibrium, substantially midway betweent-he extremes of its oscillation. For reasonably small excursions of thereed, this force apparent resonant frequency. That is, the apparentresonant frequency in the reed will be increased as compared with thenatural frequency in the absence of such force, and the means for itsproduction.

In Figure 3, I have provided means Afor exerting and utilizing such aforce for vibrato purposes. At 43 I illustrate an electromagnet having acore 44 shaped as shown and positioned relative to a reed I in a mannersuitable for the purpose. The reed and magnet are to be considered asconstituting parts of a master generator 24. The core 44 exerts on theferro-magnetic reed I a force tending to restore it at all times to itsposition of rest. Although this force may not be strictly proportionalto the reeds displacement from this position, yet it will be nearlyenough so for the purpose at hand. At any rate, its fundamentalcomponent will be in phase with the reed displacement; i. e., in timequadrature with the fundamental component of reed velocity. The forceexerted by the magnet will necessarily be small as compared with thatexerted by the actual elastic stiffness of the reed, since at most onewould not wish to vary the pitch over a range of much more than asemitone or six per cent in frequency.

The core 44 may simultaneously serve not only to direct magnetic flux,but also to replace the electrode 20 in Figure 1, and I have accordinglyillustrated it in Figure 3 as being connected by conductor I1 tothe'input grid terminal of amplier 32.

I may vary the frequency by any means adapted to vary the currentflowing in the winding of electromagnet 43. For example, the mechanismshown in Figure 2 would serve if elements 38 were resistors and if itwere connected in series with a source of direct current and with themagnet winding. However, I prefer to employ the electrical networkappearing in Figure 3. I may provide twelve magnet windings 43, eachmounted adjacent to one of the twelve master generator reeds I of thesixteen-foot octave. All are connected with each other, with a source ofdirect 'potential 60, and with the plate-cathode circut of a triode IUI,as indicated in the figure. The cathode is maintained at a suitablepositive potential relative to the grid, as will appear. At 91 I show aneon lamp of the type suitable for use in a relaxation oscillator. It'is supplied with direct current through a variable resistor 94, asshown. Shunted across it I show a condenser 95 and a resistor 96 inseries. The elements 94 to 91 inclusive constitute a relaxationoscillator conventional in every respect except that the presence of theresistor 96 prevents the discharge of condenser 95 through the lamp 91from taking place instantaneously, Awith a lconsequent undesirably steepwave front. If desired, a'low pass 12| of Figure l.

lter 99 may be provided to remove the higher harmonics present in theoutput f the oscillator. A blocking condenser 98 is provided to preventthe introduction of a direct potential onto the grid of tube Il.

By proper choice of the electrical magnitudes of elements 94 to 95inclusive, the oscillator is adjusted to produce a subaudiblefundamental frequency of from, say, ve to ten cycles per second. v

the insulators 3 of slightly soft rubber instead of L Bakelite), thepitch of each reed will vary periodically at the vibrato frequency. Itwill be apparent that, since all of the generators and 26 are slaveoscillators, their frequencies will also vary periodically, againprovided their reeds are not' too sharply resonant.

It may be more pleasing to provide each master reed with a separaterelaxation oscillator and tube IGI, and to adjust the severaloscillators to different vibrato frequencies. Or the C and Ct generatorsmight be varied from one relaxation oscillator, the D and Dit generatorsfrom a second adjusted to another subaudible frequency of vibrato, andso forth. This arrangement would lend a greater warmth to the musicperformed than would a single relaxation oscillator, since it would thenbe unlikely that' any two tones within a single octave which were playedsimultaneously would have the same frequency of vibrato. It is seldomthat two pitches of tones a semitone apart are sounded at the same time.

In Figure 4, I show an alternative method for enabling the reed I toeffect capacitance Variations. I have found that, if the heel of thereed is separated and insulated from an electrode I9 by a thin sheet ofmica 4t, the capacitance between reed and electrode will vary inaccordance with the reeds vibration. The reed and elec-l trode areinsulated from the casting 2 by means of Bakelite washers 3, `and may beelectrically connected in the circuit in the manner of reed In Figure 5,on the other hand, I show how l tory forces upon parts 2', 3, 41, I, and22 to f which it is attached. These forces cause a sympathetic Vibrationin the reed I, provided the latter is tuned to the same fundamentalfrequency as reed I. In consequence of this vibration, the capacitancebetween elements I" and lil" undergoes periodic variation, just as inFigure 1 To protect reed I from the electrostatic effect of the windingof magnet 2|. I have provided a shielding metallic wall 48, which isgrounded by being connected to the shield 22. This mechanical couplingabove described permits one to dispense with the driving magnet It isbelieved that the circuit for connecting up the elements of Figure 5will be obvious from a comparison with Figure l without their beingillustrated in detail in Fgure 5. Whereas in Figure 1 a given primaryreed actuates a secondary reed tuned to twice its fundamental frequencydue to frequency doubling at the pickup electrodes, in Figure 5 theassociated primary and secondary reeds must obviously be tuned to thesame frequency. To permit closer coupling between reeds, casting 2 maybe of aluminum.

I have found that there is a possibility of suicient electrical leakageoccurring between reeds I and I over the surface or through the body ofthe intervening supporting insulators .'i to interfere in certain caseswith the proper behavior of the device of Figure 5 unless precautionsare taken to prevent this. Thus, reed l' has at al1 times duringoperation a charge relative to ground, whereas reed I is to be chargedonly when its associated tone is to be played, as explained inconnection with Figure 1. Any leakage between reeds would charge reed I"so that its tone might sound continuously even when no key of thekeyboard were pressed. To avoid this, I have provided a grounded me alwasher 'l, which I term a trap electrode, in the leakage path betweenreed-s to intercept `and conduct to ground such leakage as mightotherwise iiow between active electrodes I and I. A similar leakagebetween any of the reeds and associated electrodes in Figure l mightinterfere with proper operation there were it not for the fact that suchleakage currents are intercepted by grounding the castings 2, 2', and 2"through the screws by which their respective reeds are attached to them,as illustrated. This means for preventing leakage between a vibratileelement and associated conducting elements, such as another reed or anelectrode is novel, so far as I am aware. The assembly screws areinsulated from reeds I and I" by sleeves around them, as previouslyexplained in connection with Figure l.

In Figure 1, I showed one means for limiting the amplitude of vibrationof a reed or other vibratile element; namely, the cOntact I2 andassociated parts. In Figure 6, I show an alternative means ioraccomplishing this end, which involves a novel shaping and positioningof the pickup electrodes calculated to eiect a reduction in electrontube system input transduction factor with increasing reed amplitude, aswill appear.

In Figure 6, the electrode I9 performs the same function as it does inFigure 1. Parts 5, I2, I5, and I8 of Figure 1 are omitted from Figure 6,and their function is assumed by electrode I9a. Electrodes I9 and iSdare connected together electrically, as shown, and in a sense constitutea common pickup electrode system. These two electrodes are bothadjustable relative to the reed and are insulated from the casting 2,just as electrode I9 is in Figure 1. Other electric connections are tobe carried out identically in the two figures. It has been consideredunnecessary to illustrate these common features in de-` tail in Figure6.

The two electrodes, being both connected to an input terminal ofamplifier 32, supply thereto an alternating potential having a complexwave form; that is, upper harmonics, through conductor I'I. It will beobserved that the phase of the fundamental component of potentialcontributed by electrode ISa will be one-hundredand-eighty degrees outof phase with that contributed by electrode I9. For small excursions ofthe reed, the potential from electrode I9 will considerably exceed thatfrom electrode I9a, because the effective area of the former is by farthe greater o the two; enough greater, in fact, to overcome the effectof the somewhat greater air gap which exists between electrode I9 andthe reed I when the latter is in its position of rest. Electricalconnections being properly carried out, as well understood, thefundamental component of potential contributed by electrode I9 may beregenerative, and that of electrode I9a may be degenerative. Since theregenerative eiect will predominate for small excursions, the reedamplitude will build up for a time. Ultimately, however, as theamplitude increases, the degenerative effect of electrode I9a willbecome nearly as great as the regenerative effect due to the relativemagnitudes of the two air gaps as above mentioned.

When the reed amplitude has attained a magnitude such that the totalfeedback forces at fundamental frequency become only just suicient toovercome the mechanical losses in the reed, the amplitude cannot furtherincrease, and a steady state as regards amplitude is established. Thus,due to wave form distortion of a special sort, the type of generator ofFigure 6 may be made self-limiting in amplitude, just as is that ofFigure 1. I have found by experiment that such self-limiting actionactually occurs provided the proper adjustments are made in themechanical and electrical elements in the device.

Experiment has also demonstrated that, under such conditions, there is astrong second partial present in the potential fed to the amplifier 32by electrodes I9 and ISa. 'I'his partial, like the second partialcreated by electrode 20 in Figure 1, may be made effective to drive thereed I.

A mathematical-study of arrangements of this type will reveal that anyarrangement and shaping of electrodes which can impart to the input ofamplifier 32 a fundamental component of potential, the ratio of whoseamplitude to that of the reed displacement increases sufficiently lessrapidly than does the reed amplitude will limit that amplitude to asmall enough value to prevent its striking the electrodes. Severalalternative constructions having this property appear in Figures 7 to 10inclusive. In each of the disclosures of Figures 6 to 10, likewise inthat of Figure 17 to be discussed later, it will appear that the reeditself forms one electrode oi' a condenser, and that it has a denitecapacitance with respect to at least one of the fixed electrodes or tothe entire assembly cf fixed electrodes. It will also be evident thatthe reed electrode and fixed electrode or electrodes are so shaped andso placed relatively that the input transduction factor which theycontribute to their associated feedback channel decreases withincreasing reed amplitude.

In order that the reed may be self starting, it is important in Figures'7 to 9 that the bottom edge of electrode 49 be about opposite themiddle of the end of the reed l, as illustrated. It will be seen thatthe capacitance between reed and electrode will reach a maximum for somesmall upward excursion of the reed, and will become less as the upwardexcursion becomes greater than this, because the reed tip is travelingin a curved path, which increases the air.` gap between reed andelectrode after the reed has risen to a certain point. Due to theshielding effect of the grounded electrode 50, the capacitance betweenreed I and electrode 49 may be expected to be quite small for reedexcursions downward from the equilibrium position shown. The resultantwave form of capacitance for sinusoidal reed vibration may be expectedto be markedly non-sinusoidal, and to provide the properamplitude-limiting action. Experiment has borne this out. The wave formhas also been found to containamarked second harmonic capable of drivingthe reed I as explainedin connection with Figures 1 and 6 above.Electrode 49 may thus be connected to conductor II in the manner ofelectrodes I9 and 20 in Figure 1, and may replace these electrodes andalso the contact type of amplitude limiter of the foregoing gure.

Figure 8 is similar to Figure '7, except that the grounded electrode 50has been omitted. Its shielding action has been found to be unnecessaryunder certain adjustments of air gaps and electric circuit elements.

Experiment has also shown that, by shaping the end of the reed I on abevel as shown in Figure 9, the reed may be made to start itself morereadily. The reason for this is not precisely known.

If there is any difliculty in starting the oscillation of the reeds inany of the disclosed devices, it will be found that oscillation mayusually be initiated by opening and suddenly reclosing the output platecircuit of amplifier 32, or by administering a mechanical shock to thereed. Either of these measures will usually shock excite the reedsufficiently so that it will maintain itself in continuous vibrationelectrically, and adjust itself automatically to its proper amplitude.It goes without saying, however, that the reed Vibration should never besufficient to cause it to strike any of the electrodes in any of thedisclosed generators. When the reeds have once attained their steadystates, they should be kept relatively free from shock, since experimenthas shown that this may disturb the amplitude-limiting mechanismmomentarily and may cause loss of control sufficient to cause contactbetween reeds and electrodes.

In Figure 10, I show other shapes and arrangements of electrodes whichmay be made to provide self-limitation of amplitude, together with a.second partial or second harmonic of potential adequate to drive anotherreed, such as reed I in Figure 1, tuned to double the frequency of reedI. Electrodes I9 and 5I in Figure 10 will feed to the amplifier 32 anon-sinusoidal potential of the proper wave form, provided they areshaped as shown, and provided their distances from the reed are properlyadjusted, as determined by experiment. In view of the foregoingdiscussion, it is believed that further elucidation of Figure 10 isunnecessary, since the performance is qualitatively the sam as that inFigure 7. Y

It will be evident that mechanico-electric translating means other thanelectrostatic ones might be used if properly placed to produce therequisite production of harmonics. For example, electromagnets havingcores placed and shaped like the electrodes shown here would doubtlessperform similarly.

In Figures 6 to 10, the limiting action is brought about vdue to thespecial shaping and positioning of the electrodes such that theircapacitance to the reed is a non-linear function of the displacement ofthe latter, and that the wave form is one which changes in the propermanner with increasing rced amplitude. In Figure l1, there appears apurely electrical means for accomplishing the same function, employing anon-linear resistance; that is, one such that the potential drop acrossit is a non-linear function of the current through it. At I are tworeeds like those similarly designated in the foregoing figures, and atI9 are their associated pickup electrodes. If these'electrodes were tobe directly connected to the input of amplifier 32, and if the currentsfed back to their magnets (as shown in Figure 1 but not in Figure 11)were sufficient to start the reeds in vibration, it would be found thatthe amplitudes of reed vibration would build up until one or the otherof the reeds struck its associated electrode. Such wide excursions maybe prevented -by employing the thermionic diodes 56, potential source60, and adjustable resistors 51 connected to conductor I'I and thence tothe input of amplifier 32 as depicted. Due to the source 60, thecathodes of the diodes conduct no current unless potential is suppliedfrom the reed sufficient to overcome this bias. The circuit is similarto that of the delayed diodes used for automatic volume control in manyradio receivers. For small reed amplitubes, the potential peaks attainedby electrodes I9 will be insuflicient to overcome the delay biascontributed by source 60. Thus, provided the feedback action driving thereeds is correct in magnitude and phase, the amplitude of either of thereeds will continue to build up until the diode associated with thatreed conducts current at the peaks of potential.

During periods of conduction, the diode partially short-circuits itsassociated electrode I9 to ground. It is prevented from greatlyinfluencing the potential of any of the other electrodes I9 which arenot associated with it, due to the decoupling action of its associatedresistor 51. The presence of the resistors also enhances theshortcircuiting action. The fundamental component of potential presentat electrode I9 is affected by the short-circuting action in such amanner as to set a limit to its magnitude, and therefore to that of thereed which it controls. It is to be noted that the diodes are not a partof the feedback amplifier 32, but that each is part of an individualfeedback channel. They perform no amplifying function whatever, theiraction being purely a limiting and wave form-distorting one. Due to thisdistortion, a second partial or harmonic appears at the input and outputterminals of amplifier 32, which may serve to drive reed I in the mannerexplained above.

All of the amplitude-limiting means described above operate by reducingthe ratio between the amplitudeof the fundamental component present inthe input potential to the feedback amplifier 32 and the amplitude ofthe fundamental component of velocity present in the resonant element(the reed). This ratio between amplitudes I term the electron systeminput transduction factor, for the sake of brevity. In the cases disfcussed, the limiting of feedback occurs between the reed and theamplifier input terminals. Hence the appropriateness of the termadopted.

There is another type of device capable of limiting theindividualamplitudes of the' several resonant elements in a somewhat differentmanner from the above. This device appears in Figure 12. In this case,the output terminals of amplifier 32 feed the voice coil of aloudspeaker cone 5,4 through the'intermediary of an output transformer55. The speaker will also have a the chest.

sume that one of the reeds I, shown in end view,

has been started in vibration in some way, as by a mechanical shock.Being connected to source through a conductor 21 and the metal top ofwind chest 5I, the reed is held above ground potential as in foregoingfigures. In consequence of the vibration of the reed, an alternatingpotential :at the fundamental frequency of the reed is fed to inputterminals of amplifier 32 in the manner explained hereinabove. A currenthaving the same fundamental frequency accordingly appears in the voicecoil of the cone 54, and the cone is maintained in vibration thereby.The rim of the cone is permanently screwed to the bottom of the windchest 5I. 'Ihis chest may be a box made of wood except for its top,which is of metal. Its bottom has a suitably large hole over the cone sothat the pressure in the chest varies periodically in accordance withthe vibration of the cone.

It is to be understood that there may be twelve reeds, one tuned to eachof the twelve tones of the sixteen-foot octave, as in Figure l. Ofthese, only two are illustrated as representative of the group. Theremaining ten may be located to the right of the two, or behind them inthe drawings. All of the electrodes I9 may be connected to the input ofamplier 32 in the manner of the two shown. I have shown a portion of thetop of the chest 5I as broken away. This was merely toavoid showing morereeds than necessary for an adequate disclosure, and I prefer to havethe chest entirely closed olf from the outside air except for suchorifices as occur around the reeds and perhaps around the speaker cone.This will aid the cone in establishing the greatest possible variationsin air pressure within the chest for a given amplitude of conevibration.

Since al given reed I is subjected on its lower face to an alternatingair pressure, it will tend to be vibrated thereby. Provided feedback issufficient in magnitude, and provided phase relations are correct forregenerative feedback as above explained, the reed may be maintained incontinuous vibration at its natural or resonant frequency.

Each of the reeds consists of a tongue and a frame, being constructedprecisely like the reeds of vthe well-known harmonium. However, the tipsof the tongues are bent somewhat differently than is common in theharmonium, so that they occupy a position relative to their frames asillustrated in the figure. Since the reeds need not be subject tomagnetic forces, they may be of brass, like harmonium reeds.

In the upper part of the wind chest I have provided a plurality of reedcells 52, each having a reed mounted in its upper wall, and each havingan orifice 53 through which it receives an alternating supply of airfrom the main body of Except for its orifice and for the slots betweenthe tongue and frame of its reed, each cell is completely closed.

For small excursions of vibration of any given reed, the above-mentionedslots will be quite small in cross section, and will provide but littleopportunity for the escape of air. However, for sufficiently largeexcursions, these slots will be seen to become quite large. Provided theorice 53 is made sufficiently small, it will provide an appreciableacoustic impedance to the alternating current of air passing through itbetween its associated cell and the main body of the wind chest. Thus,for large excursions, the amplitude of the fundamental component ofalternating air pressure in the cell will be reduced relative to that inthe main body of the chest. This will serve to limit the amplitude ofreed vibration, as Will be evident.

Provided the volume of air in the main body of the chest is made largeeno-ugh relative to that of each of the cells, and provided the acousticimpedance of each of the orifices is made suiciently great, the severalreeds will be. ef-

fectively decoupled from one another, and they u driving reeds of theeight-foot octave, as in Fig- I ure 1. To accomplish this, one need onlymount, adjacent to each of the reeds I, an additional. electrode like 5|in Figure 10, and connect the same to conductor |1 as in that figure.

I have not shown electrodes 5| in Figure l2. because to do so wouldcrowd the drawings and because the method of mounting and connectingthem should be evident in the light of foregoing figures. In Figure 12 Ihave, however, shown conductors 2l and 21' which conduct current fromsource 60 to groups of magnets 2| and 2|' respectively, in the mannerof 1. I believe that, in View of Figures 1 and 12, it will be clear thatone may combine into a single network a plurality of self-maintained ormaster reeds I which are acoustically coupled to the output of theelectron tube system and acoustically limited, and which in turnelectromagnetically drive slave reeds I and I, being coupled theretothrough the intermediary of a common amplifier 32.

In contradistinction to the limiting means of foregoing figures. that ofFigure 12 performs its limiting function by reducing the ratio betweenthe amplitude of the fundamental component f' present in the resonantelement and the amplitude of the fundamental component present in theoutput potential from the amplier 32. This yratio I term the amplifieroutput transduction factor or electron tube system output transductionfactor. That is, there is a reduction in the efficiency of couplingbetween the sound translating device 54 and the vibratile element. Inother words, the coupling which is reduced is that between the electrontube system output terminals and the reed or resonant vibratile element.

In Figures 13 and 14 appear alternative means for actuating secondaryreeds I" from actuating amplifier 32. Element 29 is an attenuator, and30 and |01 are coacting pedal-operated contacts for expression control,as known in the art. Continuously present in the output of amplifier 32are a plurality of alternating potentials, each having the frequency ofone of the reeds I or I', as seen from Figure 1. A conductor 28supplying such potentials appears both in Figures l and 14. The directpotential also present at the output is stopped oi from the attenuatorby blocking condenser 58. A resistor |02 may be included if needed toprevent the attenuator from robbing current from magnets 2| and 2|',which are also to be connected to conductor 28 in Figures 1 and 18.

When the performer presses a keyboard key |06, Figure 13, thus closingkey contact |04 to conductor |05, a connection is established fromattenuator 29 via contacts 30 and |01, conductor |05, key spring |04,decoupling resistor |03, and elements |3a and 6a to a magnet 2|. Onlyone such connection is illustrated, and that one quite conventionally,since details of the keying and stop circuits per se do not form a partof this application, but such circuits are claimed only generally and incombination with other parts. The same keying connections can evidentlybe applied to Figure 1 for the same purpose. The keying and stopcircuits of a number of other applications or patents may be employed inthe present environment, if adapted along lines evident to those workingin the art. See, for example, Re. 19,702 to Bourn, Figures 1 to 3, or mycopending application S. N. 379,287y filed February 17, 1941, Figures 1to 3. It will be understood that the electrical magnitudes of thecircuit elements present in these references may have to be altered tosuit present needs. In particular, condensers present in the citedreferences may have to be reduced in capacitance or eliminated entirelyto adapt the networks to alterhating currents.

In Figure 1 of my earlier application, it will be seen that thepolarizing potential is supplied to the stators of alternators IA to ILthrough conductors I3a to |30 and resistors 6a to 6c. These sameconductors and resistors appear with the same reference numbers in thepresent Figure 13. However, in the present instance, instead of directcurrents, they carry alternating currents supplied by ampliiier `32, ashas already been stated. Also, instead of being connected to groundthrough a resistor 6 and condenser 1, as in application 379,287, theyare connected in the present Figure 13 through the windings of magnets2|".

A study of the two applications will reveal that, when the appropriatekey and stop contacts are closed, a plurality of alternating currentshaving the frequencies of musical tones will flow in a selected one ofthese magnet windings. There will also be present a direct currentsupplied from source 60, Figure 13. The magnitudes of these currentswill be controllable by propel` key and stop manipulation the manner setforth in the earlier application. In View of all this, it is believedunnecessamr to provide vthe present application with a drawing anddeailed description of the entire complicated network.

In Figure 13. reeds I are mounted adjacent to magnets 2|", as in Figure1y each reed being tuned to a tone of the musical gamut, es eX- plained.Being sharply tuned, each reed will respond to that frequency in itsmagnet to which it is tuned, but not appreciably to any other presenttherein, and the steady state amplitude attained by the reed will beneary proportional to the amplitude of the component to which it istuned. In other words, reed amplitude (rather than reed polarizingpotential as in Figure 1) will be controllable by the manipulation ofselectors such as the keys and stops mentioned in the previousparagraph.

Adjacent to each reed I is a pickup electrode I 9". This passes on toamplifier 66 an alternating potential nearly proportional to reedamplitude, in the manner explained in connection with Figure l. Theoutput potentials of reeds Il are thus utilized in a common networkcomprising an amplifier 55.

Continuously, whether keys are pressed or not, amplifier 32simultaneously amplifies the plurality of frequencies originating atprimary sources 24 and 25. When current is admitted to a magnet byclosure of a key contact, the alternating actuating potential andcurrent therein will attain their steady states in a comparatively shortinterval of time. However, the amplitude of reed oscillation whichresults from the current in a given. magnet 2 l of a secondary source orgenerator 25 need not build up to its proximate steady state in soshort; an interval, but may be made to build up gradually in the mannercharacteristic of the pipe organ if desired, Iby proper choice of reeddamping, as will be well understood. See Patent No. 1,929,027 toMiessner, page l, lines 65 to 77. In like fashion, the decay of the tonemay also be rendered organlike.

In the foregoing figures, I have shown a -plurality of mechanicallyresonant or oscillatory elements, each tuned to a different individualfrequency of a musical scale, and each provided with individual meansfor automatically limiting the amplitude of its fundamental component,all of the resonant elements being maintained in continuous vibration bymeans of a common electron tube system. I may also employ in a similarfashion a plurality of electrically resonant elements, each providedwith its individual limiting means and all being maintained incontinuous electrical vibration by a common amplifier.

Figure 15 shows a portion of such a network. Elements 69 and 'Ill arerespectively a variable condenser and the secondary winding of anironcored audio frequency transformer. In series with each secondarywinding is a small incandescent lamp 'I3 having a tungsten filament.Through conductors 16, the two tuned tank circuits are connected tovariable resistors 51 and thence to the grid of an amplifying triode 90,just as in Figure 11. The triode is to be regarded as illustrative of amore complex amplifier having more than one stage, although in certaininstances one stage will doubtless prove adequate. The plate of thetriode shown, or the plate of that which constitutes the last stage inany case, is connected in series with the primary windings of thetransformers 'I4 and thence through the source of B potential E to thecathode of said triode. Of course I may use as many tank circuits as arenecessary for the several tonal pitches required, and all may beconnected in the manner illustrated.

The several electrically oscillatory or resonant elements, comprisinginductances and condensers, are to be loosely coupled to the platecircuit of the triode by a suitable choice of turns ratios and couplingcoefficients of the transformers 14. They are also to be loosely coupledto the grid and to each other by making the resistances of resistors 51suciently large. Nevertheless, the phase and magnitude of feedback areto be such as to maintain all of the tank circuits in continuouselectrical Vibration or oscillation, as will be understood from theforegoing discussion.

It is to be remembered that that portion of the network wherein issimultaneously present potential at the several generated frequencies isto be as free from non-linear distortion as possible; Accordingly, thetube 95 is to be operated at all times below the point of overload, andits type and operating potentials should be chosen in accordance withthis requirement.

As in foregoing disclosures, there must be means individual to each ofthe several generated frequencies which serves to limit the amplitude ofits associated frequency, and which is controlled substantially entirelyby the amplitude of a potential or current at its associated frequency.In Figure 15, such means is found in the lamps 13. It is a well-knowncharacteristic of a tungsten filament that its electrical resistanceincreases markedly with increasing temperature, particularly in certaintemperature ranges. In other words, its current and voltage arenonlinearly related when one considers the values of each averaged overlong periods such that the lament temperature has time to change,because the temperature is a function of the current in the filament.For a given potential impressed by the tube on the primary winding ofone of the transformers 14, the potential which its associated tankcircuit supplies to the grid of the tube will depend upon the resistanceof the associated lamp; the greater the current in the lamp, the hotterit will become, and the less will be the grid potential. It will be seenfrom this that if, by adjusting a resistor 51, the feedback to itsassociated tank circuit be made only a little more than suicient toinitiate oscillation with the filament cold, the increasing temperatureand resistance of that lament as oscillation builds up may serve tolimit the amplitude of current in the tank to a value which will notoverload the tube 90 or any other pari-l of the network wherein there ispresent energy associated with a plurality of different frequencies.

It is to be understood that a given tube 95, if used in a musicalinstrument, need not coact with all the tuned circuits of thesixteen-foot octave. If desired, the tones may be divided into two ormore groups such that a given tube does not serve to maintain any twoadjacent semitones, after the manner above explained in connection withreed oscillators.

In Figure 15, the lamp resistance does not change materially over asingle alternating current cycle, due to thermal lag, and there is thusno production of second partials for producing forced oscillations intuned circuits of octave frequency. Hence every tuned circuit presentsliould comprise its own amplitude-limiting lamp In Figure 16, Iillustrate another means for in--f` dependently limiting the amplitudeof energy on potential associated with each of the several fundamentalfrequencies produced by the multi-v frequency generator. Whereas inFigure 15 the non-linear resistance used as a limiting means consistedof a lamp in series with a leg of the tank or tuned circuit, in Figure16 such means or resistance consists of a diode 5S shunted across thetank circuit. As in Figure 11, there may be one such diode associatedwith each of the resonant or frequency-determining elements. In Figure,11, the frequency-determining means is the reed I. In Figure 16, it isthe combination of elements 69-14. Without illustrating a plurality ofdiodes and tank circuits in Figure 16, it is believed that the operationof the diodes will be clear in the light of Figures 11 and 15. Each ofthe diodes 56 may be connected to one end of an associated individualresistor 51, and the several resistors may be connected at their otherends to one another and the amplifier input termlnal, as in Figure l1.The primaries of the several transformers 14 may be connected in serieswith the output circuit of the amplifier, as in Figure 15.

With some reduction in the number of reeds necessary as compared withthe disclosure of Figure 1, I may employ high frequency energy in themaintenance of continuous reed vibration, as shown in Figure 17. At 68appears a source of supersonic potential, coupled through ahighfrequency transformer 61 to a tuned circuit consisting of thesecondary winding of that transformer and a tuning condenser 69. Throughcondensers 6 I, the tuned circuit is able to supply high frequencycurrent to the reeds I and I. This current returns through a portion ofeach of the potential dividers 6 to ground and thence to the tunedcircuit. By virtue of the potential drop in these portions of thepotential dividers, each of the reeds will have a high frequencypotential differing from that of ground.

Assume one of the reeds I to be in vibration. Through the capacitivepaths between the reed and the adjacent electrodes I9, and via theconductors I'| and condensers 63, high frequency current will besupplied to the variable condenser 69 located in about the center of thefigure. Another high frequency path from the reed to the same condenserexists, via the capacitive path between the reed and the electrode 49,thence through condenser 62, one of the conductors I1, and one of thecondensers B3.

Connected across the condenser 69 are a grid lea-k 9| and the inputterminals of a conventional radio frequency amplifier 10. The grid leakmay be replaced by an inductance if desired, and condenser 69 adjustedfor resonance at the frequency of the source 68. Receiving highfrequency potential from that amplifier is a conventional demodulator'II which supplies audio potential at the frequency of the reed I to theinput terminals of audio amplifier 32. Together, amplifiers 19 and 32and demodulator |I comprise an amplifying electron tube system connectedat its output to driving magnets 2| and 2|.

An inspection of the network will reveal that it thus far differsessentially from certain foregoing disclosures of this application onlyin that the reeds or resonant mechanical elements I and I are polarizedwith high frequency or supersonic potential instead of with directpotential, and that a demodulator and radio-frequency amplifier areadded to complete the feedback network. It will also be understood that,in consequence of the feedback channel via radio frequency amplifier,demodulator, audio frequency amplifier, and driving or actuatingmagnets, the reeds I may be maintained in continuous vibration oroscilla-tion'by proper attention to phase and magnitude of feedbackfactor. In the manner of foregoing figures, particularly Figures 7 tol0, the variation of capacitance between the reeds I and theirassociated electrodes |9 and 49 will be such as to limit reed amplitudeand to produce a strong second harmonic or partial for the maintenanceof vibration in reeds In Figure 1, the reeds I" could be polarized froma keyboard through resistors 6a to 6c. A similar polarization can occurin Figure 117, except that in this case it is the reeds I and which maybe thus polarized. Taking one of the reeds I for the purpose ofexplanation, suppose it to have been thus polarized. So far as direct oraudio currents are concerned, the condenser is to all intents andypurposes connected to ground at one of its terminais, and thus mayserve a purpose identical with that in Figure l. The condensers 62 areassumed to be of sufiiciently small capacitance so that they will notefficiently transmit audio frequency currents, although they may beefficient paths for currents of high frequency. In the discussion whichfollows, the electrodes 49 may accordingly be disregarded. The audiopotentials present at the two electrodesl9 will be degrees out of phasewith each other in consequence of their opposite positions with respectto the reed I.

I wish it to be understood that the two electrodes I9 are to beidentically constructed and to be adjusted to positions equidistant fromthe rest or equilibrium position of the reed. 'I'heir potentials willconsequently not only be degrees out of phase at the fundamentalfrequency, but they also will be equal in magnitude and identical inwave form. The two potentials are supplied respectively to the two gridsof a double triode 64, as shown, and the plate circuits of this triodeare connected in conventional lpush-pull fashion to the input terminalsof an amplifier 66. The amplier, in turn, feeds a loudspeaker 13.

Other reeds may supply tones of their respective frequencies to the sameamplifiers and loudspeaker, as will appear from the figure. Through theuse of high frequency feedback means for the maintenance of continuousreed oscillation, I have enabled the network of Figure 17 to deliverselected tones directly from the primary reeds I and I, without thenecessity for secondary reeds I as in Figure 1.

In Figure 18, I show a reed assembly wherein the actuating magnet 2|, byvirtue of its placement relative to the reed I, effects an automaticlimitation of the reed amplitude. 'I'he reed I, through a connection notshown in Figure 18, is to be kept charged by connection to the source60, just as in Figure 1. The tip 44 of the magnet core serves its usualpurpose of a directing flux, and also as a pickup electrode, as inFigure 3. Due to its symmetrical placement relative to the rest positionof the reed; that is, midway between the extremes of reed oscillation,only even harmonics or partials of the reeds fundamental frequency willappear at the conductor As in Figure l, this conductor is to beconnected to the input of amplifier 32 and the winding of magnet 2| isto be connected to its output.

By suitably choosing the number of stages of amplification in amplifier32, the magnitudes of the coupling impedances at the amplifier input andoutput, the type of coupling between its stages, and perhaps othernetwork parameters, the phase of the maxima of force exerted by themagnet upon the reed may be shifted at will, as will be evident to oneskilled in the communication art. 'Ihere will be two such maxima percomplete oscillation of the reed, due to the doubling of frequency justmentioned. These maxima should occur while the reed is moving toward itsposition of rest, one occurring when it is above that position andmoving downward, the other occurring when it is below that position andmoving upward. In both cases, the force will be such as to partially orentirely overcome the damping forces of the reed, and will thereforetend to maintain the reed in Vibration. In other words,

they will decrease the effective damping of the reed, although they mayalso tend to affect its frequency of vibration slightly. To prevent thefrequency shift from being unduly great, the reed should be as sharplytuned as possible.

IfV such forces are correct in phase and sufficient in magnitude, thereed amplitude will continue to build up as hereinbefore explained. Itwill be seen, however, that, the greater the reed excursion carries itfrom the tip of the magnet, the less effective will be the magneticforces upon it, at least beyond a certain critical excursion. Likewise,the less will be the interelectrode capacitance between the pole tip andthe reed tip. In other Words, the less'will be both the electron tubesystem input transduction factor and also its output transduction factoras above dened. By suitably shaping these two tips, and by properlyadjusting the phase and magnitude of amplification factor, as bestarrived at by experiment, the reed amplitude may be made to limit itselfto a critical value.

By varying the direct current in the winding of magnet 2| at a slow,subaudible frequency, a vibrato effect may be produced in the disclosureof Figure 18, as described in connection with Figure 3.

I have shown that the output potential of amplier 32 will contain astrong second harmonic of fundamental reed frequency due to thefrequency-doubling properties of the pickup electrode 44 in Figure 18.rIhis second harmonic may be made to drive an octave-tuned reed I asirl-Figure 1 by connecting the driving magnet 2l for that reed to theoutput terminals of amplifier l 32 in the manner of Figure 1. Reed l andmagnet 2| are to be placed relative to each other as illustrated in thatligure. Further reeds I and I" may be driven from the output of theamplier in the fashion shown in the same figure. It is believed thatthis will be well understood without illustration in full in Figure 18.Of course, if the amplitude-limiting scheme of Figure 18 is employed,that of Figure 1, utilizing the contact l2, will be unnecessary.

Classified from one point of view, the amplitude-limiting means of thisapplication fall into two groups; namely, those which operate by virtueof a variation in the amplier input transduction factor, and those whichoperate by vir- Y tue of a variation in the amplifier outputtransduction factor with increasing amplitude in the resonant element.In the rst class fall the means illustrated in Figures l, 6 to l1inclusive, 15, 16, and 1'7. In the latter class are the means of Figure12. The disclosure of Figure 18, it will be recalled, typifies bothtypes of variation.

One might also classify the limiting means from a second viewpoint,namely; (a) those of Figures 1 and 15, wherein the transduction factoris reduced without distortion of wave form of output potential, and (b)those wherein distortion accompany such reduction as in the gures otherthan 1 and 15.

In Figures 1, 3 to 13 inclusive, 17, and 18, the resonantfrequency-determining element is a reed, although it may clearly bereplaced by any of anumber of types of mechanically tuned members, asstated hereinabove; in Figures 15 and 16, on the other hand, theresonant frequencydetermining means comprise an inductance andcapacitance, and the tuning is electrical in nature. In either case, thewave form of oscillation in such frequency-determining means, whethersuch oscillation or vibration be electrical or mechanical, is quiteaccurately sinusoidal, due to the resonant properties just referred to,provided, of course, that the amplitude is not permitted to become toogreat. On the other hand, the wave forms of current in the network apartfrom the frequency-determining means is, or may be, markedlynon-sinusoidal. In fact, in the sustaining amplier or electron tubesystem 32, which is a part of the sustaining network, there are presenta plurality of different frequencies, because the amplier is common to aplurality of different resonant frequency-determining elements, each of`the latter being adapted for the continuous generation of a differentfundamental frequency. Also included i-n the network as a whole are aplurality of amplitude-limiting means, each associated with a differentone of the frequency-determining elements, and with a different one ofthe fundamental frequencies, as will be evident. All the above variantsare comprehended within the scope and spirit of my invention.

In accordance with my invention, primary oscillatory reeds or otherresonant elements of octaves higher than the rst (in the embodimentillustrated, octaves higher than the sixteen-foot) octave are notself-driven or self-maintained, but are maintained in continuousoscillation at their respective natural frequencies by means ofharmonics or partials intentionally set up by generators of some loweroctave. When employed in a musical instrument, this insures a perfectoctave relationship between the pitches or frequencies of tones of thesame name in different octaves, such as those of pitch G in the sixteen,eight, four, and two-foot octaves,.for example, without the necessity 0ftuning the resonant elements of any octave except the lowest. This is adistinct advantage from the standpoint of maintenance of such aninstrument. However, if the resonant means for the octaves above thelowest get seriously out of proper tune, as might occasionally happen,their amplitudes will become less, due to their departure from resonancewith the periodic force which is actuating them. This will be especiallytrue if they are very sharply resonant. It would thus seem advisable notto make the reeds or tuned circuits of the upper octaves too sharp, butto somewhat increase their damping if need be, by the addition ofmechani- Cal or electrical resistance. Electrical resistance may beprovided by inserting a resistor in the tuned circuit. Mechanicalresistance might be increased, for example, by making the insulatingwashers 3 of slightly soft material, such as a semi-hard rubber, insteadof Bakelite as above recommended, or by adding damping varies, as isWell known in the art.

In the even-tempered musical scale, the pitch of some of the harmonicsor partials of one tone will lie Very close to that of other tones. Forexample, the third harmonic of sixteen-foot C differs but slightly infrequency or pitch from the fundamental of eight-foot G, and the fifthpartial of sixteen-foot C but slightly from the fundamental of four-footE. Such harmonics, if sufliciently prominent in the electron tubesystem, might interfere with the proper operation of the resonant meansof such higher octaves, causing frequency instability and interlockingas discussed in an earlier part of this specification. For example, inFigure l, it might be expected that the reeds l and l" tuned toeight-foot G would behave erratically because they would respondsimultaneously to the second partial of sixteen-foot G and to the thirdpartial of sixteenfoot C. However, it is not essential to the properoperation of the device that any partials of a given reed or otherresonant means except the second partial be present in the amplifieroutput. Hence, amplifier overloading, which might produce unwantedpartials, is to be avoided. Moreover, the adjustments, shapes, andpositioning of electrodes such as I9 and 20 must be made such as toprevent too great a prominence of such unwanted partials, as will beWell understood. Besides, the magnetic circuits of the magnets 2 I and2l should be rather open, so as to prevent the development of suchharmonics other than fundamental and second, as well as other sum anddifference frequencies, as pointed out above. The amplitude-limitingmeans of Figures l and 15 do not function by the development of harmonicdistortion, and thus I regard them as preferable from the standpoint ofundesirable interference between generators. In Figure 1, for example,only even harmonics need be present in a strength comparable with thatof the fundamental, and the second can be made by far the most prominentof these by suitably shaping and mounting the electrodes as best learnedfrom experiment.

'I'hose self-maintained resonant or tuned oscillatory elements whichappear in all figures except Figure 18, whether such elements areelectrically or mechanically oscillatory, are maintained in continuousoscillation by producing in the amplifier 32 a potential having afrequency which is the same as the fundamental frequency of theoscillatory element. In Figure 18, on the other hand, the frequency inthe amplifier is twice that fundamenta1 natural frequency, and a halvingof frequency occurs at the air gap between the actuating magnet 2i andthe reed I. I consider, however, that there is a unity of principle inall such disclosures, because in all of them the frequency in thesharply resonant or oscillatory element bears a harmonic relationship tothat in the electron tube system. Thus, in Figure 18, the frequency inthe tube system is the second harmonic or partial of the fundamentalfrequency of the oscillatory element. In the other disclosures, thepotentials in the tube system which serve to maintain oscillation havethe same frequency as the natural frequency of the oscillatory element;that is, the tube system frequency is the first harmonic or partial ofthe fundamental frequency of the oscillatory element. I have phrasedcertain of the claims in accordance with a recognition of this unity ofprinciple; whether a the harmonic in question is the first or second, aharmonic relationship nevertheless exists.

Many of the details of my invention are applicable to other forms cf theparts with which they are associated. For example, in a given musicalinstrument, some of the vibratile elements might be reeds and some mightbe tuning forks or bars or tuned electric circuits, as will be evident,without departure from the spirit of my invention.

It will also be evident that forms of reed actuation other thanelectromagnetic might be employed within the spirit of my invention as,for instance, piezoelectric means. Similarly, pickup means might be ofthe photoelectric or electromagnetic form. Whatever the form ofactuation or pickup, however, it is essential that individual limitationof amplitude be provided for those tuned elements which maintainthemselves in oscillation.

Where the claims include a source of potential, I do not mean to confinemyself to a single source, since it is obvious that a plurality ofpolarizing sources or sources of alternating potential are equivalent ingeneral to a single source, and that separate polarizing potentialsources may be used for the several oscillatory elements Without theexercise of further invention.

Where a claim or claims include selectors, electric connections eachclosable to connect, or the like, I mean to include switches couplers,stops, keys, or similar instrumentalities except as further limited bythe language of the claim. Except when further limited, as by the wordconductively, or the like, I also do not mean to be limited to deviceswhich make electrical contacts such as a switch. Such devices may bringelectrical parts into inductive relationship, and the electricalinfluence through such selectors may be via a solid or fiuid path.

In the claims in which I use the term network or circuit, unless furtherlimitations appear, I mean to cover all conductors, or electricalnetworks, however complex, in which there can be an appreciable transferof electric charges by metallic, galvanic, ionic, or electronicconduction; I mean to cover also such instrumentalities as condensers,transformers, and gaseous, vacuum, and photoelectric tubes.

Wherever I use the term electron tube, I mean to comprehend tubes orvalves of either the gaseous or vacuum type.

Where I refer to a feedback channel or the like, unless furtherlimitations occur, I mean to include channels which are purelyelectrical, partially mechanical and partially electrical, partiallyacoustic and partially electrical, and so on, and there may be atranslation from electrical into mechanical or acoustic oscillation andback again into electrical oscillation within such a channel.

Unless further limitaitons appear, where I recite means for couplingtogether, I mean to to include mechanical, electrical, or other meansadapted to the purpose specified.

'I'he drawings and description illustrate and describe what I nowconsider to be preferred forms of the device for production ofalternating currents or potentials for any purpose, by way ofillustration only, while the broad principle of the invention will bedefined by the appended claims. In these claims, Where reference is madeto a source cf alternating potential or the like, it is not to beinferred that the production of such potentials is the ultimate purposeof the instrumentality referred to. The ultimate purpose may, forexample, be the production of musical tones.

I claim:

1. A multi-frequency oscillator, comprising: an amplifying electron tubesystem; having an input circuit and an output circuit; a plurality ofnon-linear feedback channels each electrically connected to said inputcircuit and each, in cooperation with other parts of said oscillator,maintaining in said system continuous electrical oscillation at a singlefundamental frequency and each, by virtue of its non-linearity,generating in said system an upper harmonic of its fundamentalfrequency, the oscillations maintained by the several channelscoexisting simultaneously and having frequencies which differ from eachother; constituting a part of each of said channels, an element sharplyresonant to substantially the frequency of its associated upper harmonicand maintained in oscillation by the energy in said channel; alsoconstituting a part of each of said channels, individual means limitingthe amplitude of those oscillations in said system which are maintainedby the channel which comprises said means, each said-individualmeans-being actuated only by oscillations having a harmonic relationshipto those maintained by that channel of which that means is a part.

Y 2. A multi-frequency oscillator, comprising: an amplifying electrontube system having an input circuit and an output circuit; a pluralityof nonlinear feedback channels each electrically connected to said inputcircuit, each, in cooperation With other parts of said oscillator,maintaining in said system continuous electrical oscillation at a singlefundamental frequency, and each, by virtue of its non-linearity,generating in said system an upper harmonic of its fundamentalfrequency, the oscillations maintained by the several channelscoexisting simultaneously and having frequencies which differ from oneanother; constituting a part of each of said channels, an elementsharply resonant to substantially the frequency of an upper harmonicgenerated by its associated channel and maintained in oscillation by theenergy in said channel; also constituting a part of each of saidchannels, individual means limiting the amplitude of the oscillationsmaintained by its associated channel at the input terminals of saidsystem, said means also decreasing, as said amplitude increases, theratio between that amplitude and the amplitude of the oscillations ofcorresponding frequency present in its associated channel, each saidindividual means being actuated only by oscillations having a harmonicrelationship to those maintained by its associated channel.

3. A multi-frequency oscillator, comprising: an amplifying electron tubesystem having an input circuit and an output circuit; a plurality ofnonlinear feedback channels each electrically connected to said inputcircuit, each, in cooperation With other parts of said oscillator,maintaining in said system continuous electrical oscillation at a singlefundamental frequency, and each, by virtue of its non-linearity,generating in said system an upper harmonic of its fundamentalfrequency, the oscillations maintained by the several channelscoexisting simultaneously and having frequencies Which differ from oneanother; constituting a part of each of said channels, an elementsharply resonant to substantially the frequency of an 'upper harmonicgenerated by its associated channel and maintained in oscillation by theenergy in said channel; also constituting l a part of each of saidchannels, individual means limiting the amplitude of the oscillationsmaintained by its associated channel at the output terminals of saidsystem, said means also increasing, as said amplitude increases, theratio between that amplitude and the amplitude of oscillations ofcorresponding frequency present in its associated channel, each saidindividual means being actuated only by oscillations having a harmonicrelationship to those maintained by its associated channel.

Y 4. A multi-frequency oscillator, comprising: a source of polarizingpotential; an electron tube amplifier having an input circuit an outputcircuit; a plurality of electro-mechanical non-linear feedback channelseach electrically connected to said input circuit and each, incooperation with other parts of said oscillator, maintaining in saidamplifier continuous electrical oscillation at a single fundamentalfrequency and each, by virtue of its non-linearity, generating in saidsystem an upper harmonic of its fundamental frequency, the oscillationsmaintained by the several channels coexisting simultaneously and havingfrequencies which differ from each other; constituting a part of each ofsaid channels, mechanico-electric translating means actuated by energyfrom said output circuit and a mechanically oscillatory element sharplyreson- `ant to substantially the frequency of its associated upperharmonic, each said element being maintained in oscillation by theoscillations in its channel; and, associated with each of said channels,an individual pair of electric contacts opened and closed solely by theoscillatory element in their associated channel in response to theoscillations thereof, each said pair being electrically connectedintermediate said source and their associated translating means andcontrolling the polarizing potential of their associated translatingmeans only.

5. A multi-frequency oscillator, comprising: an amplifying electron-tubesystem having an input circuit and an output circuit; a plurality ofnonlinear feedback channels each electrically connected to said inputcircuit, each, in cooperation with other parts of said oscillator,maintaining in said system continuous electrical oscillation at a singlefundamental frequency, and each, by virtue of its non-linearity,generating in said system an upper harmonic of its fundamentalfrequency, the oscillations maintained by the several channelscoexisting simultaneously and having frequencies which differ from oneanother; constituting a part of each of said channels, an elementsharply resonant to substantially the frequency of an upper harmonicgenerated by its associated channel and maintained in oscillation by theenergy in said channel; also constituting a part of each of saidchannels, individual mechanico-electric translating andamplitude-limiting means limiting the amplitude of the oscillationsmaintained by its associated channel at the input terminals of saidsystem, said means effecting such limitation by decreasing, as saidamplitude increases, the ratio between that amplitude and the amplitudeof mechanical oscillations present in its associated channel, each saidindividual means being actuated only by the mechanical oscillationspresent in its associated channel.

6. A multi-frequency oscillator, comprising: an amplifying electron-tubesystem having an input circuit and an output circuit; a plurality ofnonlinear feedback channels each electrically connected to said inputcircuit, each, in cooperation with other parts of said oscillator,maintaining in said system continuous electrical oscillation at a singlefundamental frequency, and each, by virtue of its non-linearity,generating in said system an upper harmonic of its fundamentalfrequency, the oscillations maintained by the several channelscoexisting simultaneously and having frequencies which differ from oneanother;

constituting a part of each of said channels, a mechanically oscillatoryelement, and individual mechanico-electric translating andamplitudelimiting means comprising at least tWo electrodes of acondenser Whose capacitance is periodically varied by and only by theoscillations of that oscillatory element which forms a part of its ownchannel, said electrodes being so shaped and so placed relative to eachother that the rate of change of their mutual capacitance with respect

